stern

demanding, exacting

uncharitable, unforgiving

adamant, callous, hardened, hard-hearted, hard-line, immovable, implacable, inflexible, merciless, pitiless, relentless, rock-ribbed, stiff, unbending, uncompromising, unrelenting, unsparing, unyielding

dour, gruff

ascetic (also ascetical), monastic, monkish

browbeating, bullying

bleak, cold, hostile, inhospitable, inimical, unfriendly, unsympathetic

adamant, bound, determined, firm, intent, purposeful, resolute, resolved, steadfast, unflinching

fixed, hard, hardened, hardheaded, immovable, implacable, inflexible, ironhanded, mulish, obdurate, obstinate, rigid, self-willed, set, stiff, stubborn, unbending, uncompromising, unrelenting, unyielding, willful (or wilful)

immutable, unchangeable

black, cheerless, dark, gloomy, glum, joyless, melancholic, moody, morose, sulky, sullen, surly

brooding, grave, humorless, melancholy, serious, sober, sobersided, solemn, somber (or sombre), staid, unsmiling